Underfill coating for LOC package

ABSTRACT

An LOC die assembly is disclosed including a die dielectrically adhered to the underside of a lead frame. An underfill material is introduced between each lead finger and semiconductor die, extending from the bonding location of the die and the edge of the die, in order to prevent filler particles from lodging between the leads and the active surface of the die during transfer molding of a plastic encapsulant. The seal created by the underfill material reduces point stresses on the active surface of the die usually caused by the filler particles. The decreased flexure in the leads further enhances the locking of the leads in position with respect to the die.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a "leads over chip" (LOC) semiconductor dieassembly and, more particularly, to a method and apparatus for reducingthe stress resulting from lodging of filler particles present in plasticencapsulants between the undersides of the lead frame leads and theactive surface of the die and improved lead locking of the leads inposition over a portion of the active surface of a semiconductor dieassembly.

2. State of the Art

The use of LOC semiconductor die assemblies has become relatively commonin the industry in recent years. This style or configuration ofsemiconductor device replaces a "traditional" lead frame with a central,integral support (commonly called a die-attach tab, paddle, or island)to which the back surface of a semiconductor die is secured, with a leadframe arrangement wherein the dedicated die-attach support is eliminatedand at least some of the leads extend over the active surface of thedie. The die is then adhered to the lead extensions with an adhesivedielectric layer of some sort disposed between the undersides of thelead extensions and the die. Early examples of LOC assemblies areillustrated in U.S. Pat. No. 4,862,245 to Pashby et al. and U.S. Pat.No. 4,984,059 to Kubota et al. More recent examples of theimplementation of LOC technology are disclosed in U.S. Pat. Nos.5,184,208; 5,252,853; 5,286,679; 5,304,842; and 5,461,255. In instancesknown to the inventors, LOC assemblies employ large quantities orhorizontal cross-sectional areas of adhesive to enhance physical supportof the die for handling.

Traditional lead frame die assemblies using a die-attach tab place theinner ends of the lead frame leads in close lateral proximity to theperiphery of the active die surface where the bond pads are located,wire bonds then being formed between the lead ends and the bond pads.LOC die assemblies, by their extension of inner lead ends over the die,permit physical support of the die from the leads themselves, permitmore diverse (including centralized) placement of the bond pads on theactive surface, and permit the use of the leads for heat transfer fromthe die. However, use of LOC die assemblies in combination with plasticpackaging of the LOC die assembly has demonstrated some shortcomings ofLOC technology as presently practiced in the art.

One of the shortcomings of the prior art LOC semiconductor dieassemblies is that the tape used to bond to the lead fingers of the leadframe does not adequately lock the lead fingers in position for the wirebonding process. At times, the adhesive on the tape is not strong enoughto fix or lock the lead fingers in position for wire bonding as the leadfingers pull away from the tape before wire bonding. Alternately, thelead fingers will pull away from the tape after wire bonding of thesemiconductor die but before encapsulation of the semiconductor die andframe, either causing shorts between adjacent wire bonds or the wirebonds to pull loose from either the bond pads on the die or lead fingersof the frame. While wire bonding fixtures may be used to attempt toovercome these problems, the fixtures and their use add cost to thefabrication process. Additionally, if large amounts of tape are used tofix the lead fingers in place, the reliability performance of thepackaged device will be affected as tape absorbs moisture from thesurrounding environment, causing problems during encapsulation andpotential corrosion problems.

After wire bonding the semiconductor die to the lead fingers of the leadframe, forming an assembly, the most common manner of forming a plasticpackage about a die assembly is molding and, more specifically, transfermolding. In this process (and with specific reference to LOC dieassemblies), a semiconductor die is suspended by its active surface fromthe underside of inner lead extensions of a lead frame (typically Cu orAlloy 42) by a tape, screen print or spin-on dielectric adhesive layer.The bond pads of the die and the inner lead ends of the frame are thenelectrically connected by wire bonds (typically Au, although Al andother metal alloy wires have also been employed) by means known in theart. The resulting LOC die assembly, which may comprise the framework ofa dual-in-line package (DIP), zig-zag in-line package (ZIP), smalloutline j-lead package (SOJ), quad flat pack (QFP), plastic leaded chipcarrier (PLCC), surface mount device (SMD) or other plastic packageconfiguration known in the art, is placed in a mold cavity andencapsulated in a thermosetting polymer which, when heated, reactsirreversibly to form a highly cross-linked matrix no longer capable ofbeing re-melted.

The thermosetting polymer generally is comprised of three majorcomponents: an epoxy resin, a hardener (including accelerators), and afiller material. Other additives such as flame retardants, mold releaseagents and colorants are also employed in relatively small amounts.

While many variations of the three major components are known in theart, the focus of the present invention resides in the filler materialsemployed and its effects on the active die surface and improved leadlocking of the lead fingers of the frame.

Filler materials are usually a form of fused silica, although othermaterials such as calcium carbonates, calcium silicates, talc, mica andclays have been employed for less rigorous applications. Powdered, fusedquartz is currently the primary filler used in encapsulants. Fillersprovide a number of advantages in comparison to unfilled encapsulants.For example, fillers reinforce the polymer and thus provide additionalpackage strength, enhance thermal conductivity of the package, provideenhanced resistance to thermal shock, and greatly reduce the cost of thepolymer in comparison to its unfilled state. Fillers also beneficiallyreduce the coefficient of thermal expansion (CTE) of the compositematerial by about fifty percent in comparison to the unfilled polymer,resulting in a CTE much closer to that of the silicon or galliumarsenide die. Filler materials, however, also present some recognizeddisadvantages, including increasing the stiffness of the plasticpackage, as well as the moisture permeability of the package.

Another previously unrecognized disadvantage discovered by the inventorsherein is the damage to the active die surface resulting fromencapsulant filler particles becoming lodged or wedged between theunderside of the lead extensions and the active die surface duringtransfer molding of the plastic package about the die and the inner leadends of the LOC die assembly. The filler particles, which may literallybe jammed in position due to deleterious polymer flow patterns and flowimbalances in the mold cavity during encapsulation, place the active diesurface under residual stress at the points of contact of the particles.The particles may then damage the die surface or conductive elementsthereon or immediately thereunder when the package is further stressed(mechanically, thermally, electrically) during post-encapsulationhandling and testing.

While it is possible to employ a lower volume of filler in theencapsulating polymer to reduce potential for filler particle lodging orwedging, a drastic reduction in filler volume raises costs of thepolymer to unacceptable levels. More importantly, if the volume of thefiller in the encapsulating polymer is reduced, as more polymer is used,the reliability of the encapsulated part is affected as the polymertends to absorb moisture, and is more permeable to moisture therebycausing a variety of problems for the encapsulated part duringencapsulation and subsequent use. Currently available filler technologyalso imposes certain limitations as to practical beneficial reductionsin particle size (currently in the 75 to 125 micron range, with thelarger end of the range being easier to achieve with consistency) and inthe shape of the filler particles. While it is desirable that particlesbe of generally spherical shape, it has thus far proven impossible toeliminate non-spherical flakes or chips which, in the wrong orientation,maximize stress on the die surface.

Ongoing advances in design and manufacturing technology provideincreasingly thinner conductive, semiconductive and dielectric layers instate-of-the-art dice, and the width and pitch of conductors servingvarious purposes on the active surface of the die are likewise beingcontinually reduced. The resulting die structures, while robust andreliable for their intended uses, must nonetheless become morestress-susceptible due to the minimal strength provided by the minutewidths, depths and spacings of their constituent elements. The integrityof active surface die coats such as silicon dioxide, doped silicondioxides such as phosphorous silicate glass (PSG) or borophosphoroussilicate glass (BPSG), or silicon nitride, may thus be compromised bypoint stresses applied by filler particles, the result beingunanticipated shortening of device life if not immediate, detectabledamage or alteration of performance characteristics.

The aforementioned U.S. Pat. No. 4,984,059 to Kubota et al. doesincidentally disclose several exemplary LOC arrangements which appear togreatly space the leads over the chip or which do not appear to providesignificant areas for filler particle lodging. However, such structuresmay create fabrication and lead spacing and positioning difficulties.

In addition to solving the problems associated with filler particlelodging and damage, it is desirable to have improved lead locking of thelead fingers of the frame during operations involving the semiconductordie. If the gaps between the lead fingers and the semiconductor die aresealed with an underfill material, the adhesive used to bond the leadfingers to the semiconductor die is more effective in locking the leadfingers in position. Furthermore, the use of an underfill material, inaddition to the tape, screen print or spin-on dielectric adhesive layer,provides an additional stabilizing means to immobilize the lead fingersin position, thus reducing or eliminating localized stress failuresoccurring during the transfer molding process. Previously, improvinglead finger locking has been approached from the perspective of improvedadhesives and by increasing the flexibility of the lead fingers, ratherthan sealing the gaps between the leadfingers and the semiconductor die.

From the foregoing, the prior art has neither provided for improvedlocking of the lead fingers to the semiconductor die, nor recognized thestress phenomenon attendant to transfer molding and the use of filledencapsulants, nor provided an LOC structure which beneficiallyaccommodates this phenomenon.

SUMMARY OF THE INVENTION

The present invention provides a lead-supported die assembly for an LOCarrangement that substantially reduces the stress that may otherwisepotentially form between the leads and the active die surface due to thepresence of filler particles of the polymer encapsulant and improvedlead locking of the leads in position over a portion of the activesurface of a semiconductor die assembly. Accordingly, an underfillmaterial is introduced in the gap between each lead finger andsemiconductor die, between the bonding location of the die and the edgeof the die, to underfill and seal the gap. After the underfill materialis cured, the compound filler is prevented from flowing into the gaps.Accordingly, a stacking of filler particles in which the fillerparticles try to force the lead away from the die thus causing stress inthe connection between the lead and the die is prevented or reduced.Moreover, the underfill material substantially immobilizes the leadfingers and reduces the stress created during the transfer moldingprocess as well as other processes. The resulting exclusion of fillermaterial from the gap will effectively eliminate or reduce the residualstress typically experienced by the active die surface in conventionalLOC assemblies. This lessened residual stress is carried forward in theencapsulated package after cure, permitting the package to betterwithstand the stresses of post-encapsulation handling and testing,including the elevated potentials and temperatures experienced duringburn-in, without adverse effects. The resulting lead stability alsoimproves lead finger locking to the tape as less force is transferred tothe tape from the lead fingers, which force may cause the lead fingersto become dislodged therefrom prior to the wire bonding operations or,subsequently, during encapsulation of the assembly.

The LOC apparatus of the present invention comprises a lead frame towhich the active surface of a die is adhered by a LOC tape, permittingthe lead frame to physically support the die during pre-encapsulationhandling and processing, such as wire bonding. The gap found between thelead finger and the semiconductor die is sealed with an underfillmaterial. With such an arrangement, intrusion of filler particlesbetween the inner lead ends and the active surface of the die during theencapsulation process is effectively prevented.

Stated in more specific terms and on the scale of an individual lead andthe underlying active surface of the die, an underfill material isintroduced onto the semiconductor die at a location near the leadfinger. More specifically, the underfill material may be introducednearby the lead axis between the location of the dielectric adhesive(such as LOC tape, screen print or spin-on, as known in the art)disposed on the underside of a lead and the edge of the die. Theunderfill material will migrate into and fill the gap by way ofcapillary action. The underfill prevents filler particles from flowinginto the gaps so as to substantially eliminate the stress created byfiller particles wedged or lodged between the lead finger and the die.The underfill also enhances the stability of the free end of the leadfinger, so as to immobilize the lead finger during the die assemblymolding process.

Although the die assembly and method of assembly of the presentinvention have been described in relation to several preferredembodiments, it is believed that major advantages of the assembly andmethod according to the invention are sealing the gaps between the leadfingers and the semiconductor die in order to prevent the lodging ofdamaging filler particles, and immobilizing the free ends of the leadfingers so as to eliminate localized stress failures resulting duringthe encapsulation process and during post-encapsulation handling andtesting. These and other features of the present invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying drawings, and as defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a flow chart of an exemplary process sequence forplastic package molding;

FIGS. 2A and 2B are side schematic views of a typical transfer molding,showing pre-molding and post-molding encapsulant positions;

FIG. 3 shows a top schematic view of one side of a transfer mold ofFIGS. 2A and 2B, depicting encapsulant flow and venting of the primarymold runner and the mold cavities wherein the die assemblies arecontained;

FIGS. 4A, 4B and 4C depict encapsulant flow scenarios for a mold cavity;

FIGS. 5A and 5B depict cross-sectional side views of prior art packagedSOJ semiconductor devices;

FIGS. 6A and 6B depict cross-sectional side views of a an embodiment ofa packaged SOJ semiconductor device according to the present invention;

FIGS. 7A and 7B depict cross-sectional side views of another embodimentof a packaged SOJ semiconductor device according to the presentinvention;

FIGS. 8A and 8B depict top views of a lead frame according to thepresent invention;

FIG. 9A depicts a partial cross-sectional end view of a semiconductordie assembly wherein a first method of dispensing underfill is used;

FIG. 9B depicts a partial cross-sectional end view of a semiconductordie assembly wherein a second method of dispensing underfill is used;

FIG. 10 depicts a partial cross-sectional side view of the packaged SOJsemiconductor device of FIG. 7A;

FIG. 11 depicts a partial cross-sectional side view of the embodiment ofa packaged SOJ semiconductor device of FIG. 7A;

FIG. 12 depicts a partial cross-sectional side view of an embodiment ofa packaged SOJ semiconductor device of FIG. 7A; and

FIG. 13 depicts a partial cross-sectional side view of anotherembodiment of a packaged SOJ semiconductor device of FIG. 7A.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

So that the reader may more fully understand the present invention inthe context of the prior art, it seems appropriate to provide a briefdescription of a transfer apparatus and method for forming a plasticpackage about an LOC die assembly. The term "transfer" molding isdescriptive of this process as the molding compound, once melted, istransferred under pressure to a plurality of remotely-located moldcavities containing die assemblies to be encapsulated.

FIG. 1 is a flow chart of a typical process sequence for plastic packagemolding. It should be noted that the solder dip/plate operation has beenshown as one step for brevity; normally plating would occur prior totrim and form.

FIGS. 2A and 2B show pre-molding and post-molding positions ofencapsulant during a transfer molding operation using a typical moldapparatus comprising upper and lower mold halves 10 and 12, each moldhalf including a platen 14 or 16 with its associated chase 18 or 20.Heating elements 22 are employed in the platens to maintain an elevatedand relatively uniform temperature in the runners and mold cavitiesduring the molding operation. FIG. 3 shows a top view of one side of thetransfer mold apparatus of FIGS. 2A and 2B. In the transfer moldapparatus shown, the encapsulant flows into each mold cavity 44 throughthe short end thereof.

In operation, a heated pellet of resin mold compound 30 is disposedbeneath ram or plunger 32 in pot 34. The plunger descends, melting thepellet and forcing the melted encapsulant down through sprue 36 and intoprimary runner 38, from which it travels to transversely-orientedsecondary runners 40 and across gates 42 into and through the moldcavities 44 through the short side thereof flowing across the dieassemblies 100, wherein die assemblies 100 comprising dies 102 withattached lead frames 104 are disposed (usually in strips so that a stripof six lead frames, for example, would be cut and placed in and acrossthe six cavities 44 shown in FIG. 3). Air in the runners 38 and 40 andmold cavities 44 is vented to the atmosphere through vents 46 and 48. Atthe end of the molding operation, the encapsulant is "packed" byapplication of a higher pressure to eliminate voids and reducenon-uniformities of the encapsulant in the mold cavities 44. Aftermolding, the encapsulated die assemblies are ejected from the cavities44 by ejector pins 50, after which they are post-cured at an elevatedtemperature to complete cross-linking of the resin, followed by otheroperations as known in the art and set forth in FIG. 1 by way ofexample. It will be appreciated that other transfer molding apparatusconfigurations, as well as variations in the details of the describedmethod, are known in the art. However, none of such are pertinent to theinvention, and so will not be discussed herein.

Encapsulant flow in the mold cavities 44 is demonstrably non-uniform.The presence of the die assembly 100 comprising a die 102 with leadframe 104 disposed across the mid-section of a cavity 44 splits theviscous encapsulant flow front 106 into upper 108 and lower 110components. Further, the presence of the (relatively) large die 102 withits relatively lower temperature in the middle of a cavity 44 permitsthe flow front on each side of the die 102 to advance ahead of the flowfront which passes over and under the die 102. FIGS. 4A and 4B show twomold cavity encapsulant flow scenarios where, respectively, the lowerflow front 110 and the upper flow front 108 lead the overall encapsulantflow front 106 in the cavity 44 containing the die assembly 100. FIG. 4Cdepicts the advance of a flow front 106 from above, before and after adie 102 is encountered, the flow being depicted as time-separated,instantaneous flow fronts 106a, 106b, 106c, 106d, 106e and 106f.

Encapsulant filler particles, as noted above, become lodged between leadends and the underlying die surfaces. The non-uniform flowcharacteristics of the viscous encapsulant flow, as described above, maycause (in addition to other phenomena, such as wire sweep, which are notrelevant to the invention) particles to be more forcefully drivenbetween the leads and the die and wedged or jammed in place inlow-clearance areas. As the encapsulant flow front advances and the moldoperation is completed by packing the cavities, pressure insubstantially all portions of the cavities reaches hydrostatic. Withprior art lead and adhesive LOC arrangements, the relative inflexibilityof the tightly-constrained (adhered) lead ends maintains the pointstresses of the particles trapped under the leads. These residualstresses are carried forward in the fabrication process to post-cure andbeyond. When mechanical, thermal or electrical stresses attendant topost-encapsulation processing are added to the residual point stressesassociated with the lodged filler particles, cracking or perforation ofthe die coat may occur, with the adverse effects previously noted. Ithas been observed that filler particle-induced damage occurs morefrequently in close proximity to the adhesive, where lead flexurepotential is at its minimum.

To graphically illustrate the above description of particle lodging,FIG. 5A depicts a prior art packaged LOC assembly wherein a single lead112 extends over a die 102, with a segment of dielectric adhesive 114,in this instance a piece of Kapton™ polyamide tape, adhered to both thelead 112 and the active surface 116 of the die. As better illustrated inFIG. 5B (DETAIL A), filler particle 130, which is part of the packagingmaterial 123, is lodged between lead 112 and die active surface 116. Itis clear that the lead end 122 is tightly constrained from movement bythe inflexibility of the attachment of the lead end 122 to the die 102by the adhesive 114. Moreover, the relative closeness of the lead 112 tothe die active surface 116 and the inability of the lead 112 to flex orrelax to reduce stress occasioned by the presence of the filler particle130 may continue even after the encapsulant has reached hydrostaticbalance such that the filler particle 130 may become tightly lodgedbetween the lead 112 and the die active surface 116.

FIG. 6A, and in better detail FIG. 6B, depicts, in contrast to the priorart, a packaged LOC arrangement according to the present invention,wherein a single lead end 122 also extends over die 102. In addition todisposing an adhesive layer 114 between the underside of the lead 112and the active surface 116 of the die 102, an underfill material 117 isapplied between the underside of the lead 112 and the active surface 116of the die 102 and between the adhesive 114 and the side 115 of the die102. As more fully set forth below (see discussion of FIGS. 9, 10, and11), various methods of underfilling may be utilized to seal the gap.The underfill 117 fills and seals the gap found between the underside oflead 112 and the active surface 116 of the die 102. Thus, a fillerparticle 130, of the same size and shape as that shown with respect tothe prior art, is prevented from entering and becoming lodged betweenthe lead 112 and the active surface 116. Moreover, the stacking of suchparticles 130 to create a similar lodging effect, also as seen in theprior art, is likewise prevented.

Additionally, because the underfill 117 (after curing) forms asubstantially rigid link between a longitudinal length of the lead 112and the corresponding active surface 116 of the die 102, the free end121 of the lead 112 is substantially immobilized and prevented fromflexing, twisting, or bending away from the active surface 116. Thus, inaddition to eliminating point stresses caused by trapped particles, theresulting relatively inflexible and tightly-constrained lead 112 reachesa steady state position before being subjected to non-uniform flowcharacteristics, which can create additional stresses such as wiresweep, during the encapsulation or molding procedure. The added leadstability afforded by the underfill 117 also reduces mechanical,thermal, and electrical stresses attendant to post-encapsulationprocessing. Furthermore, the incorporation of the underfill 117 resultsin less force being transferred to the adhesive 114 from the lead end122, said force potentially causing a dislodgement of a lead 112 fromthe adhesive 114 prior to the wire bonding operation or during theencapsulation process.

FIGS. 7A and 7B depict an alternative arrangement according to thepresent invention, wherein the adhesive 114 disposed between theunderside of the lead 112 and the active surface 116 covers a shorterlongitudinal portion of the underside of the lead 112. As in theembodiment shown in FIGS. 6A and 6B, an underfill 117 is applied to filland seal the gap formed between the underside of the lead 112 and theactive surface 116 of the die 102 and extends from the adhesive 114 tothe side 115 of the die 102. As a result of the underfill-enhanced lead112, stability and the ensuing reduction in lead-adhesive forcetransferral, a smaller amount of adhesive 114 (e.g. a narrower strip oftape) is required to adequately bond the lead 112 to the active surface116 of the die 102. Additionally, or in the alternative, due to theunderfill-enhanced lead 112 stability, a lead-die bond of sufficientstrength can be obtained through the use of an adhesive 114 of lowerstrength than that required in "traditional" LOC assemblies currentlyutilized in the industry. While the amount of underfill 117 in thepresent embodiment is proportionally increased in relation to the amountof adhesive 114 being used, this arrangement likewise prevents fillerparticles from flowing into the gaps. Also shown, are wires 151connecting leads 112 to bond pads (not shown) on active surface 116 ofdie 102.

FIGS. 8A and 8B depict views of a lead frame and associated die inaccordance with the present invention. For purposes of clarity andperspective, the inner, solid line 220 in FIG. 8A is the periphery ofthe die onto which the lead frame is superimposed and to which the leadframe is adhesively secured. In FIGS. 8A and 8B, double-dashed line 200is the outer lateral border of the plastic package to be molded on eachlead frame. In FIG. 8A, dashed line 210 represents the portion of thelead ends 122 that are typically plated. The periphery of the adhesivesegments disposed between certain leads or buses and the die arerepresented by inner, solid lines 240. For purposes of illustration, thesemiconductor die as illustrated comprises memory devices, such asdynamic random access memory (DRAM), or static random access memory(SRAM), although the invention has equal utility to any semiconductordevice wherein an LOC arrangement is employed.

FIG. 8A depicts an arrangement wherein a lead frame 150, superimposed ona die 102, is secured thereto with dielectric adhesive strips orelongated segments 152 running along each side of active die surface116. The inner lead ends 122 of the leads 112 thus extend inwardly overadhesive segments 152 toward a row of bond pads 124 running along thecenter 100 of the die 102. The inner lead ends 122 are then wirebondedto the bond pads by wires 151.

Once the inner lead ends 122 are wirebonded to the bond pads, anunderfill material is applied to the existing semiconductor dieassembly.

Referring to drawing FIG. 8B, the wires 151 connecting the leads 112 tothe bond pads 124 are shown with more clarity. As illustrated, one-halfof each bond pad 124 located along centerline 100 of die 102 is shownwith one end of a wire 151 bonded thereto.

As illustrated in FIG. 9A, which depicts a partial cross-sectional endview of a semiconductor die assembly, underfilling is accomplished byapplying the underfill material 117 along a longitudinal edge 136 of thelead 112. The underfill material 117 is applied with an underfilldispenser 132, such as a syringe having a suitable nozzle thereon or anyother dispensing means known in the art, such as sprinkling theunderfill material onto the semiconductor die 102 in the area betweenthe leads 112, dripping the underfill material onto the semiconductordie 102, spraying the underfill material onto the semiconductor die 102,write dispensing the underfill material onto the semiconductor die 102,etc. Capillary attraction between the underfill material 117 and thelead and die surfaces causes the underfill material 117 to flow into thegap between the lead 112 and the die 102. FIG. 9A illustrates, from leftto right, the flow characteristics of the underfill material 117 due tocapillary action as the underfill process is carried out.

As illustrated in FIG. 9B, which depicts a partial cross-sectional endview of a semiconductor die assembly, underfilling is accomplished bysprinkling, dripping, spraying, write dispensing, etc. the underfillmaterial 117 along a longitudinal edge 136 of the lead 112. Theunderfill material 117 is applied in suitable well known manners in theart for the sprinkling, dripping, spraying, write dispensing, etc. ontothe die 102 in the area between the leads 112. Capillary attractionbetween the underfill material 117 and the lead and die surfaces causesthe underfill material 117 to flow into the gap between the lead 112 andthe die 102.

The underfill material is typically a polymeric material, such as anepoxy or an acrylic resin. The underfill material 117 typically has athermal coefficient of expansion that approximates that of the die 102and/or the lead 112 to help minimize stress placed on either the die 102or the lead 112 during the operation of the die caused by the heating ofthe underfill material 117. To promote filling of the gap between thedie 102 and the lead 112, the viscosity of the underfill material 117 iscontrolled, taking into account the flow characteristics of theunderfill material (including adhesion and surface tension properties),the material characteristics of the die 102, the materialcharacteristics of the lead 112, and the size of the gap. Preferably,the viscosity of the underfill material 117 should be left low enough toreadily prevent the flow of the underfill material 117 significantlybeyond the perimeter of the lead 112, or in the alternative, beyond theperimeter of the die 102.

After application of the underfill material 117, the material is curedor dried by heat, ultraviolet light, radiation, or other suitable meansin order to form a solid mass.

As illustrated in FIGS. 10 and 11, the cured underfill material 117 canpossess various configurations, depending on the specific method ofunderfilling utilized and on the amount of underfill material 117 used.FIG. 10 depicts an arrangement, or first embodiment of the invention,wherein the underfill material 117 is substantially contained within thegap between the die 102 and the lead 112. Due to the forces of adhesionand surface tension inherent in the underfill material 117, the exposedsurface of the underfill material will form a lune, meniscus, orcrescent-shaped configuration. This crescent-shaped configurationprevents the formation of bubbles or air pockets that may occur due tonon-uniform encapsulant flow during the encapsulation process byeliminating angles formed in the gap.

FIG. 11 depicts an alternative arrangement, or second embodiment of theinvention, wherein the underfill material 117, in addition to fillingthe gap between the die 102 and the lead 112, substantially covers theactive surface 116 of the die 102. As previously described in FIG. 9,the underfill material 117 is applied with an underfill dispenser 132along a longitudinal edge 136 of the lead 112 or sprinkled onto thesurface of the die 102. However, in the present embodiment, an excess ofunderfill material 117 is applied onto the active die surface 116 of die102 so as to substantially cover the entire active surface of die 102.Due to the use of an overflow of underfill material, the amount ofunderfill material used and the specific placement of the underfilldispenser need not be precisely calibrated.

FIG. 12 depicts an another alternative arrangement, or third embodimentof the invention, wherein the underfill material 117 is applied on topof lead 112. Unlike the underfill process described in FIG. 9, thepresent underfill method is accomplished by applying the underfillmaterial 117 onto the top surface 134 of lead 112. A sufficient amountof underfill material 117 is applied to cover top surface 134, cover thelongitudinal edges 136 of lead 112, and fill the gap between the die 102and the lead 112. Excess underfill material 117 then flows along thelongitudinal edges 136 and fills the gap through capillary action, aspreviously described. The present embodiment further enhances thestability of the free end of the lead finger, immobilizing andencapsulating the lead 112 with underfill material, during the dieassembly molding process.

FIG. 13 depicts yet another alternative arrangement, or fourthembodiment of the invention, wherein the underfill material 117 isapplied either on top of lead 112 or on top of the lead 112 and thesurface of the die 102, thereby covering and underfilling at least aportion of lead 112, at least a portion of active surface 116 of die102, and wire 151 bonded to lead 112 and bond pad 124. Unlike theunderfill process described in FIG. 9, the present underfill method isaccomplished by applying the underfill material 117 onto the top surface134 of lead 112. A sufficient amount of underfill material 117 isapplied to cover top surface 134, cover the longitudinal edges 136 oflead 112 and fill the gap between the die 102 and the lead 112, cover atleast a portion of the active surface 116 of die 102, and cover wire151. The underfill material 117 flows along the longitudinal edges 136and fills the gap through capillary action, as previously described. Thepresent embodiment further enhances the stability of the free end of thelead finger, immobilizing and encapsulating the lead 112 as well as wire151 with underfill material, during the die assembly molding process.The packaging material 123 encapsulates the die 102 and leads 112 duringthe molding process as described hereinbefore.

Thus, it will be appreciated by those of ordinary skill in the art thatseveral different underfill coating methods and arrangements may beemployed, as illustrated, with a single lead frame and die design, toprevent compound filler particles from flowing into the gaps between theactive surface of the die and the lead. While the invention has thus farbeen described in terms of reducing the incidence of die coat damage dueto filler particles, it should also be recognized that the use of theunderfill material between the leads and the die enhances the uniformityof the flow front of encapsulant, and reduces the tendency towardformation of voids by promoting flow of the encapsulant over and aroundthe leads and over the die surface. It is believed that improved PRT(Preconditioned Reflow Test, also termed a "popcorn" test by virtue ofits deleterious effect on substandard package integrity) performance,indicative of reduced levels of moisture in the cured encapsulant, willbe realized. However, this has yet to be empirically demonstrated.

The present invention has been disclosed in terms of certain preferredembodiments as illustrated and described herein. However, those ofordinary skill in the art will recognize and appreciate that it is notso limited, and that many additions, deletions and modifications to, andcombinations of, the disclosed embodiments may be effected withoutdeparting from the scope of the invention as hereinafter claimed. Forexample, the method of underfilling may include any known orcontemplated method in the art. Various types of LOC lead frameassemblies, including multi-layer LOC lead frames such as a two-frameLOC assembly described in the above-referenced U.S. Pat. No. 5,461,255,may be adapted to the present invention. Further, the invention is notlimited to a particular arrangement of leads, or to a particular leadcross-section or configuration.

What is claimed is:
 1. A method of assembling a semiconductor die to alead frame to substantially eliminate the gap between the lead membersof said lead frame and the active surface of said semiconductor die whensaid lead frame is assembled thereto, said method comprising the stepsof:providing a lead frame having a plurality of lead members, at leastone lead member of said plurality of lead members having longitudinaledges, having a top surface, having a bottom surface, having a lead endportion connected to a portion of the lead frame, having a length,having a thickness, and having a free end portion; providing a diehaving an active surface having at least one bond pad thereon and havingat least one outer edge; providing an adhesively coated material;superimposing said lead frame on said die with said active surface lyingadjacent said lead frame and said at least one lead member of theplurality of lead members extending over a portion of the active surfaceof said die; and securing a portion of the bottom surface of the freeend portion said at least one lead member to one side of said adhesivelycoated material; securing a portion of said active surface of said dieto the other side of the adhesively coated material; and applying anunderfill material to the gap formed between the remaining unsecuredportion of the bottom surface of the free end portion of said at leastone lead member of said plurality of lead members and said activesurface of said die to substantially eliminate said gap between said atleast one lead member of said lead frame and said active surface of saidsemiconductor die when said lead frame is assembled thereto.
 2. Themethod of claim 1, further comprising the step of:curing said underfillmaterial.
 3. The method of claim 1, further comprising the stepof:drying said underfill material.
 4. The method of claim 1, furthercomprising the step of:adhering said at least one lead member of saidplurality of lead members to said active surface of said die by saidunderfill material.
 5. The method of claim 1, further comprising thestep of:adhering each lead member of said plurality of lead members tosaid active surface of said die by said underfill material.
 6. Themethod of claim 1, wherein the step of applying an underfill materialincludes:applying said underfill material substantially between eachlead member of said plurality of lead members and said active surface ofsaid die.
 7. The method of claim 1, wherein the step of applying anunderfill material includes:applying said underfill material at alocation substantially adjacent to said longitudinal edge of said atleast one lead member of said plurality of lead members.
 8. The methodof claim 1, wherein the step of applying an underfill materialincludes:applying said under fill material substantially over said topsurface of said at least one lead member of said plurality of leadmembers.
 9. The method of claim 1, wherein the step of applying anunderfill material includes:applying said underfill materialsubstantially on a portion of said active surface of said die.
 10. Themethod of claim 9, wherein a sufficient quantity of underfill is appliedto substantially cover said active surface of said die.
 11. The methodof claim 1, wherein said underfill material is applied with an underfilldispenser.
 12. The method of claim 1, wherein said underfill dispenseris a syringe.
 13. The method of claim 1, wherein said underfill materialis sprinkled on said die.
 14. The method of claim 1, wherein saidunderfill material is dripped on said die.
 15. The method of claim 1,wherein said underfill material is sprayed on said die.
 16. The methodof claim 1, wherein said underfill material is write dispensed on saiddie.
 17. The method of claim 1, wherein the step of applying saidunderfill material includes:applying said underfill materialsubstantially over a portion of said top surface of said at least onelead member of said plurality of lead members and a portion of saidactive surface of said die.
 18. The method of claim 1, wherein themethod further comprises the step of:bonding at least one wire to aportion of said at least one lead member and said at least one bond padon said active surface of said die.
 19. The method of claim 18, whereinthe step of applying an underfill material includes:applying saidunderfill material substantially on a portion of said at least one leadmember, a portion of said active surface of said die, and a portion ofsaid at least one wire.
 20. The method of claim 18, wherein a sufficientquantity of underfill is applied to substantially cover said portion ofsaid active surface of said die and a portion of said at least one wire.21. A method of transfer molding a plastic package about a die assemblyusing a molding compound including filler particles in a transfermolding machine, said method of transfer molding comprising the stepsof:disposing a lead frame having an underfill material disposed betweenthe bottom surface of a second portion of the length of at least onelead member of a plurality of lead members of said lead frame and aportion of an active surface of a die, said die being secured by itsactive surface to a first portion of said at least one lead member ofsaid plurality of lead members in a lead-over-chip configuration in amold cavity of said transfer molding machine; introducing a softenedmolding compound including filler particles therein into said moldcavity to form the plastic package; and reducing stress on the activesurface of said die attributable to contact of the filler particles inthe softened molding compound contacting the at least one lead memberand the active surface of said die by said underfill material locatedbetween the bottom surface of said at least one lead member and theactive surface of said die thereby preventing filler particles in thesoftened molding compound from flowing between said active surface ofsaid die and the bottom surface of said at least one lead member of aplurality of lead members during said transfer molding.
 22. The methodof claim 21, wherein said filler particles are prevented from flowingbetween said active surface of said die and said at least one leadmember of said plurality of lead members by underfill material.
 23. Themethod of claim 22, wherein said underfill material is substantiallylocated between said active surface of said die and said at least onelead member of said plurality of lead members.
 24. The method of claim22, wherein said underfill material is substantially located betweensaid active surface of said die and said plurality of lead members.